Self-curing, heat reactive interpolymers, prepared by copolymerizing N-alkylol amides of alpha, beta ethylenically unsaturated carboxylic acids with other comonomers, have been widely used to coat or otherwise treat paper and other fibrous products such as textiles, or nonwoven fabrics to enhance various properties including strength, tear, bonding, softness (hand), printability, flexibility, dry cleanability, and the like. These interpolymers can also be used to treat plastics, metal and wood substitutes. Evidence has shown that N-methylol acrylamide, the preferred alkylol amide used in these interpolymers, releases formaldehyde vapors at typical drying/curing temperatures for articles coated or impregnated with these interpolymer emulsions. Questions have been raised as to the safety of exposure to formaldehyde both through bodily contact with products containing formaldehyde, and with formaldehyde vapors. Thus, formulators have been seeking formaldehyde-free interpolymers.
Interpolymers containing the above-described N-alkylol amide moieties are disclosed in U.S. Pat. No. 323,533. This patent discloses acrylate based interpolymers containing preferably N-methylol acrylamide and methacrylamide moieties useful as coating and/or treating agents for paper and other fiber containing articles.
In general, self-curing interpolymers, as discussed above, and other similar latex binders are used to impart strength and dimensional stabilitY to paper, nonwoven and woven fabrics. A nonwoven fabric is a textile structure consisting of a mat of fibers held together by a binder. The fibers can be partially or completely oriented or they can be randomly distributed.
Nonwoven fabrics are popular owing to the simplicity and economy of their production since the traditional weaving operations are not used; hence, less equipment, less space, and fewer personnel are required. Nonwoven fabrics can also be produced from what would normally be considered as waste fibers, and useful characteristics are obtained which may not be provided by woven or knitted fabrics.
Enormous quantities of fibers are consumed annually in the manufacture of articles such as clothing, interliners, filters, automotive door panels, heat and electrical insulation packaging, sanitary napkins, fillers for quilted articles, wiping cloths, towels, masks, wall coverings, shoe uppers and liners, curtains and draperies, tea bags, simulated leather gaskets, luggage, ribbons, and diapers.
To obtain a nonwoven fabric with substantially uniform strength in all directions, random distribution of the fibers has been achieved by several methods. One of the most popular of such methods involves air-laying of the fibers by stripping the fibers from a carded web by means of an air stream which then directs the fibers through a restricting throat which is controlled to adjust the thickness of the resulting web.
A number of methods have been developed for treating randomly dispersed webs with a binder. Typically, a water-based emulsion binder system is used in which a thermoplastic or thermoset synthetic polymer latex is included and a loose web of fibers to be treated is immersed therein or sprayed therewith using special equipment which accommodates the structural weakness of the web. The treated web is then dried and cured to effect proper bonding. Alternatively, an aqueous or solvent solution binder system of a thermoplastic or thermoset resin may be used to impregnate the fibrous web.
The strength and dimensional stability that is required of paper, nonwoven and woven fabrics is currently achieved by bonding fibers in place with a latex that is crosslinkable. Usually, it has been known to obtain crosslinking by the copolymerization of functional or reactive monomers into the major copolymer backbone. Examples of such common commercial reactive monomers include N-methylol acrylamide, N-(n-butoxy methyl) acrylamide, N-(iso-butoxy methyl) acrylamide, N-methylol methacrylamide, and other similar materials. These monomers undergo a condensation reaction on crosslinking that evolves formaldehyde. These monomers also contain formaldehyde as an impurity. In addition to the use of these reactive monomers, crosslinking has also been attained by post-adding various aminoplasts to the latex. In these prior art situations, the use of condensates of melamine and formaldehyde has been prevalent.
On the basis of information presented above, it has been believed that the latex systems used in the past evolved formaldehyde to achieve crosslinking of the polymer solids in the latex binder and/or with the substrate. Of course, precautions have been taken to eliminate or reduce the potential hazards this represents. Nonetheless, it would be of great advantage to develop interpolymers which neither contain nor release formaldehyde.